<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="app">
    <p>{{fullName}}</p>
  </div>
</body>
<script src="../lib/vue.js"></script>
<script>
  let myVue = new Vue({
    el:"#app", 
    data:{
      firstName: "12",
      lastName: "34"
    },
    computed:{
      fullName:{
        get(){
          return this.firstName + this.lastName;
        },
        set(newValue){
          let arr2 = newValue.split(" ")
          this.firstName = arr2[0];
          this.lastName = arr2[1];
        }
      }
    }
  })
</script>
</html>